Successful operation of a laser graphics imaging device requires precise control of the focussed energy of a patterning laser beam. For the typical Gaussian cross sectional energy distribution, the minimum diameter D of the focussed spot may be determined in accordance with the expression EQU D=1.22f.lambda./d
where .lambda. is the wavelength of the laser beam, d is the diameter of the incident light beam and f is the focal length of the focussing lens.
When the wavelength of the beam employed falls within the visible range of the optical spectrum, it is not difficult to focus the beam to have a minimum diameter on the order of 1 m.mu.. As the beam is focussed to form a precisely controlled spot on an optically sensitive material, for example the planar surface of an optically sensitive plate, an extremely precise pattern can be formed by causing relative movement between the point or spot of incidence of the focussed laser beam and the optically sensitive material. A typical application of laser graphics image formation is semiconductor processing or maskless lithography in which a pattern is formed directly upon the material of interest without the use of a mask.
Because the optical mounting and support structure through which the laser beam is precisely controlled and focussed must be configured with extreme precision, relative movement between the optically sensitive material (upon which the pattern is to be formed) and the beam forming optics is normally achieved by way of an X-Y transport table upon which the optically sensitive material is mounted, with the focussing optics being stably secured in a substantially vibration free environment.
The line width of the graphics pattern to be formed by the focussed laser beam is affected by the focussed diameter of the incident laser beam, the exposure energy (intensity of the laser beam times exposure time), the .gamma.-value of the optically sensitive material and its spectral response characteristic. As a consequence, in order to obtain as precise a control as possible of these values, it is necessary to provide a mechanism through which the diameter of the pattern forming laser beam, specifically the diameter of the focussed spot in which the energy of the laser beam is concentrated on the optically sensitive material can be adjusted and monitored.
In conventional laser graphics beam focussing systems. adjustment of the characteristics of the laser beam has been accomplished by the substitution of different lenses, or by adjusting the difference between the focussing lens and the optically sensitive material, followed by adjustment of the diameter of the focussed beam. In a system where different lenses are employed to adjust the diameter of the beam, it is impossible to provide for continuous control of the beam. On the other hand, where adjustment has been accomplished by varying the distance between the focussing lens and the material upon which the pattern is formed, the ability to monitor the characteristics of the focussed beam has not been readily achievable because of the location of the focussing lens, which must be continuously adjustable.